At the present time, thermal transfer recording is widely used as a simple printing method. The thermal transfer recording can simply form various images and thus is utilized in printing wherein the number of prints may be relatively small, for example, in the preparation of ID cards, such as identification cards, photographs for business, or is utilized in printers of personal computers or video printers.
When a full-color gradational image such as a photograph-like image of a face is desired, the thermal transfer sheet used is such that, for example, various colorant layers of yellow, magenta, and cyan and optionally black are provided as colorant layers in a large number in a face serial manner on a continuous substrate sheet.
Such thermal transfer sheets are classified roughly into thermal transfer sheets of the so-called “thermal ink transfer” type wherein the colorant layer is melted and softened upon heating and as such is transferred onto an object, that is, an image-receiving sheet, and thermal transfer sheets of the so-called “sublimation dye transfer” type wherein, upon heating, a dye contained in the colorant layer is sublimated to permit the dye to migrate onto the image-receiving sheet.
When the above thermal transfer sheet is used, for example, for preparing identification cards or documents, the thermal ink transfer type is advantageous in that monotonous images, such as letters or numeric characters, can be easily formed. In the thermal ink transfer type, however, for example, gradation rendering of images is disadvantageously unsatisfactory.
On the other hand, in the case of the sublimation dye transfer type, gradational images, such as photograph-like images of a face, can be faithfully formed. Images formed by the sublimation dye transfer method, however, are disadvantageously poor in fastness or resistance properties such as weathering resistance, abrasion resistance, and chemical resistance, because, unlike images formed by conventional printing inks, the image is free from any vehicle and is formed of only dyes.
The following method is known as a method for solving the above problems. Specifically, a protective layer thermal transfer sheet having a transferable protective layer is put on top of an image formed by thermal transfer of a heat-fusion ink layer or a sublimable dye. The assembly is heated by means of a thermal head, a heating roll or the like to transfer the transferable protective layer to form a protective layer on the image. The provision of the protective layer can improve the abrasion resistance, chemical resistance, solvent resistance and other properties of the image to some extent. Further, the incorporation of an ultraviolet absorber or the like in the protective layer can improve the lightfastness of the image.
When a protective layer is transferred from the conventional protective layer thermal transfer sheet, a transferred protective layer constituting the outermost surface is flat and has excellent gloss. In general, however, this protective layer cannot meet a requirement for a matte tone in color photographs or other images. For example, the following methods are considered effective for meeting the requirement for a sublimation dye transferred image with a matte tone:
1) a method wherein a filler is incorporated in a dye-receptive layer in a thermal transfer image-receiving sheet to form fine concaves and convexes on the surface of the receptive layer;
2) a method wherein the surface of a sublimation dye transferred image is treated by an emboss roll or the like to form fine concaves and convexes on the surface of the image;
3) a method wherein a filler is incorporated in a protective layer; and
4) a method wherein, after the transfer of a protective layer, the surface of the protective layer is treated by an emboss roll or the like to form fine concaves and convexes on the surface of the protective layer.
In the method 1), however, the presence of the fine concaves and convexes on the surface of the receptive layer makes it difficult to form a high-definition image. In the method 2), the embossing is detrimental to the high-definition image and deteriorates the image quality. In the method 3), the transparency of the protective layer is lowered, and the sharpness of the image is lowered. In the method 4), the provision of one additional step is necessary for image formation. Further, since the sublimation dye transferred image is generally formed in a small lot, continuous processing is impossible. This incurs high cost, and, thus, the method 4) cannot be put into practical use.
Accordingly, an object of the present invention is to provide a protective layer thermal transfer sheet which can impart an excellent matte tone to an image, produced by a sublimation dye transfer method, without the need to additionally provide any special step and without deteriorating high definition and high sharpness of images inherent in sublimation dye transferred images.